1. Field of the Invention
The present invention relates to electronic respiratory monitoring and more particularly to an apnea detection method and apparatus for rejecting artifact and reducing false negative indications of apnea.
2. Description of Related Art
Apnea, from the Greek for xe2x80x9ca want of breath,xe2x80x9d is an interruption in breathing. Apnea or cessation of breathing may be a significant event for people of any age, but is particularly significant in a newborn. Consequences of apnea range from merely annoying to life-threatening. Various prior art schemes have been provided to monitor breathing for the purpose of detecting apnea. Pneumatic systems measure actual air flow. Others measure sounds. A more powerful diagnostic tool has been found in the electronic monitoring of breathing and heartbeat waveforms.
For example, U.S. Pat. No. 5,206,807, issued Apr. 27, 1993 and entitled Neonatal Cardiorespirograph Incorporating Multi-Variable Display and Memory, illustrates a system sensors and transducers providing signals indicative of cardiac activity, respiratory effort and relative saturation of oxyhemoglobin and transmits them to a computer. The respiratory effort may be measured by impedance pneumography. The computer produces many outputs including a real time electrocardiogram (ECG) waveform and a display of respiratory effort versus time. These two plots may be combined in a cardiorespirogram (CRG). The system provides a user-settable apnea alarm which is variable from five to 30 second intervals after a last breath. Generally, a delay in breathing over 20 seconds is considered to be an apneic event.
Reliable operation of such a system is dependent upon the pneumograph detecting the absence of a breath. In impedance pneumography for infants, electrodes are placed on an infant chest wall providing a very low current, e.g. 300 xcexcamps at 40-100 kHz. Volume changes, as for example due to breathing, are accompanied by a change in electrical resistance. A low level signal is produced indicative of chest resistance. If breathing stops and if another low level signal is produced within the delay period following a last breath, a computer may interpret this signal as having been a breath. Causes of such other low level signals include cardiac artifact and change of volume between electrodes not due to breathing activity, such as a sigh. Cardiac artifact may also take the form of high level signals. If such other low level signals are interpreted as a breath, an apneic event will have been missed, and a false negative result provided.
It is therefore a particular advantage of the present invention to provide a method and apparatus for apnea monitoring utilizing impedance pneumography in which outputs of a respiratory monitor are evaluated for behavior indicative of breathing effort.
It is a further particular advantage of the present invention to provide a method and apparatus of the type described in which cardiac artifact in a respiratory monitor signal output is not detected as respiratory event.
It is a further specific advantage of the present invention that a method and apparatus of the type described are provided in which a respiratory signal possibly indicating a respiratory event is compared to prior and successive corresponding signals for testing for cardiac artifact in the respiratory signal.
Another particular advantage of the present method and apparatus is ability to detect breaths even in view of significant amounts of cardiac artifact.
A further advantage of the present method and apparatus is the ability to resolve potential and actual respiratory events even in the presence of significantly changing or shifting levels of the respiratory signal.
It is a general advantage of the present invention to provide a method and apparatus of the type described in which signals affecting production of an apnea alarm signal are evaluated for preventing false negative results.
A more specific advantage of the present method and apparatus in which rates of changes of respiratory signals are measured so that waveforms possibly indicating a respiratory event are rejected as artifact if their characteristics do not correspond to characteristics of breathing activity.
Briefly stated, in accordance with the present invention, there are provided a method and apparatus in which ECG and electronic respiratory outputs are provided from electrodes on a patient and received for processing. A monitor examines respiratory outputs. The respiratory output, measured by impedance pneumography, will be measured in ohms. A breath begins with an inhalation, represented as a positive amplitude excursion from a minimum to a maximum. An exhalation is represented by a negative amplitude excursion A first level impedance transition is called a xe2x80x9cbump.xe2x80x9d A bump is resolved when a current maximum has been reached and the waveform decreases by at least a first, predetermined amount. Successive bumps may indicate that a breath has occurred. If the time between the current bump, i.e. the time of its maximum, and that of the previous bumps generally corresponds to the period of the heart rate, it may be concluded that the bumps were cardiogenic artifact. If they are not, they are examined to see if the impedance magnitude excursion reaches a second, higher level indicative of the industry standard of a breath. A bump manifesting this second, larger impedance transition level is called a xe2x80x9cpeak.xe2x80x9d If the current bump is a peak, it is provisionally recognized as a respiratory event, i.e. a breath. The current bump is then compared to a next bump to again determine whether the current bump and the next bump represent cardiogenic artifact. If not, a breath is detected. Rate of change criteria are used with respect to the respiratory signal to reject waveforms having successive maxima due to a baseline shift or a sigh from being reported as a breath. An alarm may be actuated when a threshold time differential between breaths is exceeded to indicate apnea. The alarm is reset in response to respiratory event. The present invention prevents artifact from forestalling production of an apnea alarm.